When a low viscosity fluid drives a high viscosity fluid, the interphase between both fluids is not flat, rather becomes unstable and creates structures called viscous fingers. In a pore, tube or channel when a viscous fluid is driven under constant flow or constant pressure gradient by a lower viscosity fluid, the driving fluid penetrates the driven fluid forming a front in the shape of a single finger within the driven fluid leaving a viscous fluid layer “glued” to the walls of the pore, tube or channel.
Fluid behavior inside tubes, Hele-Shaw cells and porous medium is described by Darcy'Law, which relates fluid pressure and velocity through fluid permeability in the medium, therefore the results obtained in tubes and Hele-Shaw cells extrapolate with minor modifications to porous media.